Optical fibers always exhibit a certain birefringence due to a difference of refractive index on two orthogonal axes (fiber birefringence axes). That difference arises both from the practical impossibility of manufacturing a perfectly circular fiber and from imperfections arising during the manufacturing process.
Birefringence causes a variation in the state of polarization of radiation propagating along the fiber. This phenomenon is characterized by a well-defined periodicity and a distance between two successive fiber points at which the propagating radiation presents the same state of polarization is called "beat length".
The knowledge of the birefringence characteristics of a single-mode optical fiber, and hence of the polarization of a radiation transmitted along the fiber, is necessary in applications of single-mode fibers where polarization is exploited. Among these applications we may cite optical communications, based on heterodyne or homodyne detection and requiring the interfering radiations (radiation containing the information and radiation emitted by a local oscillator) to have the same polarization, and optical-fiber sensors, in which fibers preserving a given state of polarization are used.
Various methods are known for detecting the local state of polarization in single-mode optical fibers and for measuring the birefringence of such fibers and, more particularly, for measuring polarization beat length in high birefringence optical fibers.
For example, "Preservation of polarization in single-mode fibers", by S. C. Rashleigh and R. G. Stolen, Laser Focus-Fiberoptic Technology, May 1983, pages 155-161, describes a method in which a fiber end is illuminated so as to equally excite both fundamental modes, the intensity maxima of the light scattered due to Rayleigh scattering are transversely observed, and the average distance of such maxima on a rather long fiber trunk is measured. Since the scattered radiation is polarized, the light emitted gives information as to local polarization and hence the calculated distance corresponds to beat length.
This method has the disadvantage of requiring a high-power source (e.g. 50-100 mW) in order to enable the scattered radiation to be detected without resorting to extremely sophisticated apparatus. Another drawback of this method is that it provides inaccurate measurements, since the exact location of said maxima is difficult to detect. Moreover, the optically inaccurate fiber cladding must be removed to allow good observation.
The need for high-power sources and fiber cladding removal is eliminated in another method described by N. Chinone and R. Ulrich in "Elasto-optic polarization measurement in optical fiber", Optics Letters, Vol. 5, No. 1, January 1981, pages 16-18.
According to that method, a transverse force is applied to the fiber and a polarization component is measured at the fiber output as a function of the force direction in a plane perpendicular to the fiber axis.
The polarization value at the force application point is derived from the value of the state of polarization measured at the fiber output. The method supplies only an indirect measurement of the local polarization, since the state of polarization changes from the point of application of the force to the fiber output.
It is then necessary to hypothesize a mathematical model of fiber polarization behavior by which the values measured at the output are to be evaluated to obtain the values of the local state of polarization. The method accuracy is bound by the precision of the hypothesized model.